In recent years, flat panel display devices have been developed and widely used in electronic applications such as personal computers. One of such devices is a field emission display (FED) device that overcomes some of the limitations of liquid crystal displays and provides significant advantages over the traditional LCD devices. For instance, the field emission display devices have higher contrast ratio, larger viewing angle, higher maximum brightness, lower power consumption and a wider operating temperature range when compared to a conventional thin film transistor liquid crystal display panel.
One of the most drastic difference between a FED and a LCD is that, unlike the LCD, FED produces its own light source utilizing colored phosphor. The FEDs do not require complicated, power-consuming backlights and filters and as a result, almost all the light generated by a FED is visible to the user. Furthermore, the FEDs do not require large arrays of thin film transistors, and thus, a major source of high cost and yield problems for active matrix LCDs is eliminated.
In a FED, electrons are emitted from a cathode and impinge on a fluorescent coating layer, such as a phosphor layer on the back of a transparent cover plate to produce an image. Such a cathodoluminescent process is known as one of the most efficient methods for generating light. Contrary to a conventional CRT device, each pixel or emission unit in a FED has its own electron source, i.e., typically an array of emitting microtips. A voltage difference existed between a cathode and a gate attracts electrons from the cathode and accelerates them toward the phosphor coating. The image produced by the phosphor coating is therefore largely dependent on the quality and the uniformity of the phosphor coating. The process for coating a phosphor layer on a transparent glass plate is therefore an important step in the total fabrication process of a FED.
Conventionally, the coating of a fluorescent powder layer on a flat transparent plate (or on a flat bulb) is accomplished by either a screen printing or a dip coating process. In either process, the screen printing paste or the dip coating emulsion contains a large amount of solvent which is used in forming the paste or the emulsion. After a paste is printed on a glass plate or after a glass plate is dip coated in an emulsion, the glass plate must be oven baked to evaporate all the solvent. The screen printing and the dip coating process are therefore not only complicated fabrication processes, but also processes that generate pollution for the environment for release of evaporated solvent into the atmosphere. The complicated fabrication process further result in a low throughput of the device fabricated.
It is therefore an object of the present invention to provide an apparatus for coating a fluorescent powder on a flat panel that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for coating a fluorescent powder on a flat panel that operates on an electrostatic coating principle.
It is a further object of the present invention to provide an apparatus for coating a fluorescent powder on a flat panel electrostatically that utilizs gas flames for heating the panel and negatively charging the panel.
It is another further object of the present invention to provide an apparatus for coating a fluorescent powder on a flat panel that is capable of rotating a flat panel mounted on a powder spray chamber during the electrostatic coating process.
It is still another object of the present invention to provide an apparatus for coating a fluorescent powder on a flat panel that is capable of heating the panel to a temperature of at least 100.degree. C. and simultaneously charging the panel with a negative voltage of at least 20,000 volts.
It is yet another object of the present invention to provide an apparatus for coating a fluorescent powder on a flat panel that is capable of electrostatically coating the powder on a panel that is electrically non-conductive.
It is still another further object of the present invention to provide a method for coating a fluorescent powder on a flat panel by simultaneously heating a glass panel to a high temperature and negatively charging the panel to a high negative voltage and directing positively charged powder particles at the panel surface.
It is yet another further object of the present invention to provide a method for coating a fluorescent powder on a flat panel by simultaneously heating, rotating and negatively charging the panel and directing positively charged powder particles under a high air pressure toward the panel surface.